Laws of motion

Photo by: Sashkin

The term laws of motion generally refers to three statements originally
devised by English physicist Isaac Newton (1642–1727) in the 1680s.
These laws, along with Newton's law of gravitation, are generally
considered to be the ultimate solution to a problem that had troubled
scholars for more than 2,000 years: motion.

History

Examples of motion are everywhere in the world around us. What makes a
rock fall off a cliff? How does a skate slide across an icy surface? What
keeps the planets in their orbits around the Sun? It is only natural,
then, that questions about motion were foremost in the minds of ancient
philosophers and physicists.

Greek philosopher Aristotle (384–322
B.C.
), for example, tried to find the causes of motion. He said that some
forms of motion were "natural." Rocks fall toward the ground
because the ground is a natural place for rocks to be. Objects rise into
the air when they are heated because the Sun is hot, and so it is natural
for heat to rise.

Aristotle classified other forms of motion as "violent"
because they were not natural to his way of thinking. For example,
shooting an arrow through space produced violent motion since the
arrow's natural tendency was to fall straight down toward Earth.

Aristotle's thinking about motion dominated Western thought for
2,000 years. Unfortunately, his ideas were not really very productive, and
scholars tried continually to improve on the concepts of natural and
violent motion—without much success.

Then, in the early seventeenth century, Italian astronomer and physicist
Galileo Galilei (1564–1642) proposed a whole new way of looking at
the problem of motion. Since asking
why
things move had not been very productive, Galileo said, perhaps
physicists should focus simply on describing
how
they move. A whole new philosophy of physics (the science of matter and
energy) was created and, in the process, the science of physics itself was
born.

Newton's three laws

Newton, who was born in the year that Galileo died, produced a nearly
perfect (for the time) response to Galileo's suggestion. He said
that
the movement of objects can be fully described in only three laws. These
laws all show how motion is related to forces. One definition for the term
force in science is a push or a pull. If you push a wooden block across
the top of a table, for example, you exert a force on the block. One
benefit of Newton's laws is that they provide an even more precise
definition for force, as will be demonstrated later.

The first law.
Newton's first law of motion is that an object tends to continue
in its motion at a constant velocity until and unless an outside force
acts on it. The term velocity refers both to the speed and the direction
in which an object is moving.

For example, suppose that you shoot an arrow into space. Newton's
first law says that the arrow will continue moving in the direction you
aimed it at its original speed until and unless some outside force acts on
it. The main outside forces acting on an arrow are friction from air and
gravity.

Words to Know

Acceleration:
The rate at which the velocity of an object changes with time.

Force:
A physical interaction (pushing or pulling) tending to change the state
of motion (velocity) of an object.

Inertia:
The tendency of an object to continue in its state of motion.

Mass:
A measure of an amount of matter.

Velocity:
The rate at which the position of an object changes with time,
including both the speed and the direction.

As the arrow continues to move, it will slow down. The arrow is passing
through air, whose molecules rub against the arrow, causing it to lose
speed. In addition, the arrow begins to change direction, moving toward
Earth because of gravitational forces. If you could imagine shooting an
arrow into the near-perfect vacuum of outer space, the arrow would
continue moving in the same direction at the same speed forever. With no
air present—and beyond the range of Earth's gravitational
attraction—the arrow's motion would not change.

The first law also applies to objects at rest. An object at rest is simply
an object whose velocity is zero. The object will continue to remain at
rest until and unless a force acts on it. For example, a person might hit
the object with a mallet. The force of the blow might change the
object's motion, giving it both speed and direction.

The property of objects described by the first law is known as inertia.
The term inertia simply means that objects tend to continue in whatever
their state of motion is. If moving, they continue to move in the same
way, or, if at rest, they continue to remain at rest unless acted on by an
outside force.

The second law.
Newton's second law clearly states the relationship between motion
and force. Mathematically, the law can be stated as
F
=
m
·
a
, where
F
represents the force exerted on an object,
m
is the object's mass, and
a
is the acceleration given to the object. The term acceleration means how
fast the velocity of an object is changing and in what direction.

To understand the second law, think of a soccer ball sitting on the
ground. If you kick that ball with a certain force, the ball will be given
a certain acceleration. If you kick the ball with twice the force, the
ball will be given twice the acceleration. If the ball then bounces off
the goal post and out of bounds, the force of the impact with the goal
post will change the ball's direction.

The second law provides a more precise way of defining force. Force is any
action that causes a body to change the speed or direction with which it
is moving.

The third law.
Newton's third law says that for every action there is an equal
and opposite reaction. A simple example of the law is a rocket. A rocket
is simply a cylindrical device closed at one end and open at the other end
in which a fuel is burned. As the fuel burns, hot gases are formed and
released through the open end of the rocket. The escape of the gases in
one direction can be considered as an action. Newton's law says
that this action must be balanced by a second action that is equal in
magnitude and opposite in direction. That opposite action is the movement
of the rocket in a direction opposite that of the escaping gases. That is,
the gases go out the back of the rocket (the action), while the rocket
itself moves forward (the reaction).